1. Field of the Invention
The present invention relates to a process for producing vinyl chloride resin, and more particularly to a process for producing vinyl chloride resin with high in polymerization productivity, the resin being high in bulk density and with less fish eye, by the use of a polymerization reactor equipped with a reflux condenser.
2. Description of the Prior Art
In the production of vinyl chloride resin it is often the case that a reflux condenser is used for improving productivity as well as for energy-saving but this is accompanied by problems in that suspension polymerization under cooling by a reflux condenser often causes an increased porosity in the particle interior and affects the smoothness of the particle's surface and the resulting deterioration of the filling property causes lowering of the bulk density and an increase of fish eyes. Moreover, the use of the reflux condenser causes a violent bubbling phenomenon in the middle stage of polymerization and thereafter. As to bulk density, it is well known that is correlated with a vinyl chloride resin's productivity in processing and lowering of bulk density is known to cause lowering of an extruder's output to thus result in reduction of productivity in processing. As means of improving a polyvinyl chloride's bulk density there is known, for instance, a method of adding vinyl chloride monomer in the course of polymerization (Japanese Laid-Open Patent Application No. 97679/75) but the vinyl chloride resin obtained by this process is known to have many fish eyes and, moreover, the residual monomer in the resin is difficult to remove.
Meanwhile, the market's need for less fish eye content in vinyl chloride resin (hereinafter referred to as "PVC") has been increasing year after year and the fish eye contact of PVC plasticized with a high polymer plasticizer of relatively low plasticizing ability and high viscosity such as of the polyester series has been a problem. In order to solve the problem of fish eye, content it is recommended to prevent formation of low-porosity particles caused by low dispersion frequency through the best possible improvement of the dispersion-coalescence frequency of monomer droplets, and also to improve the homogeneity of particles in the polymerized system by allowing local monomer addition polymerization of the monomer condensed in the reflux condenser to take place in the top layer of polymer suspension as a result of inhibiting bubbling in the middle stage of polymerization and thereafter. When a reflux condenser is used, gas generating from the monomer droplets are contained in the polymer suspension and result in lower homogeneity attainable by stirring and cause an increase in fish eye content due to a bubbling phenomenon in which polymer particles form a floating creamy layer on the polymer suspension in the middle stage of polymerization and thereafter.
Further, when the bubbling phenomenon is marked, there is caused another problem in that the polymer suspension overflows into the reflux condenser and its piping and cause deposition of scales to adversely affect the product's quality, also causing lowering of the heat-removing capacity of the reflux condenser and seriously affecting the safety control of the producing process.